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Date:   Wed, 31 Aug 2022 20:06:09 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        syzbot <syzkaller@...glegroups.com>,
        Yajun Deng <yajun.deng@...ux.dev>, netdev@...r.kernel.org,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH 1/2] netlink: Bounds-check nlmsg_len()

The nlmsg_len() helper returned "int" from a u32 calculation that could
possible go negative. WARN() if this calculation ever goes negative
(instead returning 0), or if the result would be larger than INT_MAX
(instead returning INT_MAX).

Cc: "David S. Miller" <davem@...emloft.net>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>
Cc: Paolo Abeni <pabeni@...hat.com>
Cc: Pablo Neira Ayuso <pablo@...filter.org>
Cc: Jozsef Kadlecsik <kadlec@...filter.org>
Cc: Florian Westphal <fw@...len.de>
Cc: syzbot <syzkaller@...glegroups.com>
Cc: Yajun Deng <yajun.deng@...ux.dev>
Cc: netdev@...r.kernel.org
Cc: netfilter-devel@...r.kernel.org
Cc: coreteam@...filter.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 include/net/netlink.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/net/netlink.h b/include/net/netlink.h
index 7a2a9d3144ba..f8cb0543635e 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -576,7 +576,15 @@ static inline void *nlmsg_data(const struct nlmsghdr *nlh)
  */
 static inline int nlmsg_len(const struct nlmsghdr *nlh)
 {
-	return nlh->nlmsg_len - NLMSG_HDRLEN;
+	u32 nlmsg_contents_len;
+
+	if (WARN_ON_ONCE(check_sub_overflow(nlh->nlmsg_len,
+					    (u32)NLMSG_HDRLEN,
+					    &nlmsg_contents_len)))
+		return 0;
+	if (WARN_ON_ONCE(nlmsg_contents_len > INT_MAX))
+		return INT_MAX;
+	return nlmsg_contents_len;
 }
 
 /**
-- 
2.34.1

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